There are use-cases when not only Actor, but also Invoker and even
simple RPC client must wait (e.g. sendtx or dumptx CLI commands). Actor
requires optional signers in constructor, and it's not always
appropriate to create Actor only to be able to use Waiter, sometimes
it's needed to use only Waiter without Actor.
Signed-off-by: Ekaterina Pavlova <ekt@morphbits.io>
